At present, OA devices such as a personal computer and wordprocessor have widely spread, and various printing apparatuses and printing methods for printing information input by such OA device on a printing medium have been developed. Particularly, more OA devices process color information upon improvement of the information processing ability. Along with this, printing apparatuses for outputting information to be processed are shifting to color ones.
As a printing apparatus capable of forming a color image, there are proposed various printing apparatuses in terms of the cost, function, and the like, including a low-cost printing apparatus having a relatively simple function and a multifunctional printing apparatus capable of selecting the printing speed and image quality in accordance with the type of image to be printed and the application.
In particular, ink-jet printing apparatuses have a feature of easily achieving low noise, low running cost, downsizing, and color printing. Such ink-jet printing apparatuses are widely used for a printer, copying apparatus, facsimile apparatus, and the like.
Color ink-jet printing apparatuses generally print a color image by using three colors of ink, cyan, magenta, and yellow, or four inks including black ink in addition to these inks.
Conventional ink-jet printing apparatuses generally use dedicated paper having an ink absorption layer as a printing medium in order to obtain a color image at high color development without any ink blur. At present, printing apparatuses which have printing suitability for “plain paper sheets” used in large quantities by a printer, copying apparatus, and the like upon improvement of ink are also put into practical use.
A printing means for printing in a plurality of colors such as color printing is a printhead in which printing nozzle arrays are arranged side by side. In this printhead, printing nozzle arrays (nozzles for use) for respective colors that are used for printing are sequentially arranged along the main scanning direction perpendicular to the printing medium convey direction (sub-scanning direction). In single printing/scanning, ink droplets are discharged to the same raster from nozzles.
A means for performing higher-image-quality printing in the ink-jet printing apparatus using the side-by-side printhead is high-resolution printing. An effective means for this purpose is an arrangement using a high-density printhead in which the integration densities of printing elements and nozzles of the printhead are increased. Recently, high-density printheads using a semiconductor process have been introduced. Printheads whose printing nozzle arrays have a high density of 600 dpi (nozzle pit: about 42.3 μm) are also manufactured.
For higher densities, printheads in which a plurality of printing nozzle arrays are arranged parallel to each other and offset by a predetermined amount in the sub-scanning direction are also manufactured. For example, when the density of one printing nozzle array is 600 dpi, two printing nozzle arrays are arranged parallel to each other, and offset at 1,200 dpi (nozzle pitch: about 21.2 μm) in the sub-scanning direction. This printhead can be used as a 1,200-dpi high-density printhead.
Another means for printing at higher image quality is small-droplet printing. To realize this, an arrangement using a printhead in which the printing element and nozzle of a printhead are downsized to discharge small droplets becomes effective. In recent years, printheads capable of discharging small droplets at a discharge amount of 4 to 5 pl are available. Printheads advantageous to high-resolution printing are also manufactured.
Higher-image-quality printing can be achieved using a printhead advantageous to high-density printing, or a printhead capable of discharging small droplets advantageous to higher-resolution printing.
In the use of such printhead, however, the influence of ink discharge from a plurality of printing nozzle arrays may appear. Since ink droplets discharged from nozzles draw surrounding air, an air current (air flow) is generated by movement of ink droplets upon discharging many ink droplets and at the same time, high-speed movement of the printhead. The air flow may directly influence discharge.
A mechanism of generating an air flow will be described in detail. Generation of an air flow corresponding to operation of the printhead will be explained with reference to FIG. 1.
FIG. 1 is a plan view showing the discharge surface of the printhead. Ink is discharged from a printing nozzle (not shown) in a direction perpendicular to the sheet surface. In FIG. 1, as the printhead moves in the traveling direction (main scanning direction), it discharges ink from the printing nozzles of printing nozzle array 1 to print on a printing medium. At this time, an ink flow is generated by ink discharge immediately below the printing nozzles, and acts as a “gas wall” which inhibits a gas flow. If the printhead moves in the traveling direction in this state, an air current is generated behind the gas wall, and serves as an air flow (indicated by arrows in FIG. 1). Air then flows behind printing nozzle array 1 in the traveling direction. This air flow may influence ink discharge from printing nozzle array 2.
FIG. 2 is a side view showing the printhead.
Similar to FIG. 1, as the printhead moves in the traveling direction, it discharges ink from the printing nozzles of printing nozzle array 1 to print. An air current by an ink flow behind a gas wall will be explained. As shown in FIG. 2, a downward air current (air flow) is generated by discharging ink, and the flow direction is changed backward near a printing medium.
FIG. 3 is a front view showing the printhead in the traveling direction.
FIG. 3 particularly depicts printing nozzle array 2 of the printhead shown in FIG. 1 or 2. Similar to FIG. 1, as the printhead moves in the traveling direction, it discharges ink from printing nozzles to print. Ink droplets from printing nozzles at the ends of printing nozzle array 2 are discharged inward near a printing medium under the influence of an air flow.
As a result, ink droplets discharged from printing nozzles near the two ends of printing nozzle array 2 land at positions shifted inward from original landing positions on a printing medium, and are recognized as an image error identical to a distortion or discharge failure. This image error occurs because discharge of ink droplets from printing nozzles at ends shifts under the influence of an air flow flowing behind a gas wall described with reference to FIG. 1 and an air flow generated by ink discharge described with reference to FIG. 2.
The influence of such air flow is strong when the distances between a plurality of printing nozzle arrays of the printhead are short and the printing nozzle arrays are close to each other. The influence is weak for a large ink volume, but strong for a small ink volume. Also, the influence is strong for a high printing nozzle density because the number of ink droplets which generate an air flow increases. The influence is strong for a high moving speed of the printhead. Although the degree of influence changes depending on the type of printhead, its moving speed, and ink discharge conditions, an air flow is generated around the printhead.
In a printing apparatus using a conventional printhead, an air flow occurs depending on the ink discharge state and printing conditions of the printhead, degrading the quality of an image formed on a printing medium. To solve this problem, complicated printing conditions must be performed, or control disadvantageous to printing operation must be performed by excessively decreasing the moving speed of the printhead.